Ideologies of racial purity and pollution are as old as America, and so is interracial mixing. Yet the one-drop rule did not, as many have suggested, make all mixed-race people black. From the beginning, African Americans assimilated into white communities across the South. Often, becoming white did not require the deception normally associated with racial "passing"; whites knew that certain people were different and let them cross the color line anyway. These communities were not islands of racial tolerance. They could be as committed to slavery, segregation, and white supremacy as anywhere else, and so could their newest members-it was one of the things that made them white. The history of the color line is one in which people have lived quite comfortably with contradiction.

A grandmother and granddaughter from Cape Verde. Photo by O. Louis Mazzatenta/National Geographic

And so is the past. Migration and mingling are essential to human success in the past, the present and into the future

In the future, a lot of people might look like Danielle Shewmake, a 21-year-old college student from Fort Worth, Texas. Shewmake has dark, curly hair, brown eyes, and an olive skin tone that causes many to mistake her heritage as Mediterranean. Her actual pedigree is more complex. Her father is half-Cherokee and half-Caucasian, and her mother, who was born in Jamaica, is the child of an Indian mother and an African and Scottish father.

â€˜My sister and I are just a combination of all that,â€™ she says, adding that she dislikes having to pick a particular racial identity. She prefers the term â€˜mixedâ€™.

Differences in physical traits between human populations accumulated slowly over tens of thousands of years. As people spread across the globe and adapted to local conditions, a combination of natural selection and cultural innovation led to physical distinctions. But these groups did not remain apart. Contact between groups, whether through trade or conflict, led to the exchange of both genes and ideas. Recent insights from the sequencing of hundreds of thousands of human genomes in the past decade have revealed that our speciesâ€™ history has been punctuated by many episodes of migration and genetic exchange. The mixing of human groups is nothing new.

What is new is the rate of mixing currently underway. Globalisation means that our species is more mobile than ever before. International migration has reached record highs, as has the number of interracial marriages, leading to a surge of multiracial people such as Shewmake. While genetic differences between human populations do not fall neatly along racial lines, race nevertheless provides insight into the extent of population hybridisation currently underway. This reshuffling of human populations is affecting the very structure of the human gene pool…

James Q Whitman is the Ford Foundation professor of comparative and foreign law at Yale Law School. His subjects are comparative law, criminal law, and legal history. His latest book is Hitlerâ€™s American ModelÂ (2017).

On 5 June 1934, about a year and half after Adolf Hitler became Chancellor of the Reich, the leading lawyers of Nazi Germany gathered at a meeting to plan what would become the Nuremberg Laws, the centrepiece anti-Jewish legislation of the Nazi race regime. The meeting was an important one, and a stenographer was present to take down a verbatim transcript, to be preserved by the ever-diligent Nazi bureaucracy as a record of a crucial moment in the creation of the new race regime.

That transcript reveals a startling fact: the meeting involved lengthy discussions of the law of the United States of America. At its very opening, the Minister of Justice presented a memorandum on US race law and, as the meeting progressed, the participants turned to the US example repeatedly. They debated whether they should bring Jim Crow segregation to the Third Reich. They engaged in detailed discussion of the statutes from the 30 US states that criminalised racially mixed marriages. They reviewed how the various US states determined who counted as a â€˜Negroâ€™ or a â€˜Mongolâ€™, and weighed whether they should adopt US techniques in their own approach to determining who counted as a Jew. Throughout the meeting the most ardent supporters of the US model were the most radical Nazis in the room…